Sigma-Delta based digital to analog converters (D/A) are mostly used to convert digital signals to analog signals for playback on the earpiece transducer or loudspeaker of a mobile phone. The advantage of these types of converters is that their over-sampling nature allows quantization noise to be reduced to lower levels than with other converters, as well as benefiting from integrated circuit (IC) lithography feature size reductions. These converters, in general, may produce a low-level background noise and other artifacts due to non-linear behaviors. This is the effect of limit cycle oscillations that results in the presence of periodic components in the output. These components may translate into low-level correlated audio tones, possibly random in amplitude that will be audible to the person using the phone, especially during periods of silence.
Attempts in the past to alleviate the problem described above, also known as spurious tone artifacts, have included dithering techniques or adding an out-of-band sine or square wave dither that would involve complicated generation of such dither as well as complicated filtering out of the dither. In one instance, dithering with pseudo-random white noise is effective if the dither signal is shaped according to the quantization noise transfer function of the modulator where the dither amplitude is intended to be relatively high.